我如何从另一个班级收到实例？

Question

I have public class Human , where located instance private int age; Also, I have public class Student extends Human , which accordingly inherits Human . Also, I have class Group .

public class Group implements Comparable<Group>  {
    private Student[] group = new Student[10];
}

I want to sort students by age private int age .

How can I receieve the instance age of the class Human or Student ? I have smth like that now:

@Override
public int compareTo(Group o) {
    return o.getAge - this.getAge;
}

As you can realize, I have this error:

getAge cannot be resolved or is not a field

Answer 1

What you could do to fix this:

First, you have private field which is accessible only inside its class. In your case, you could add public methods to get/set value to make it accessible to the outside world.

public class Human {
    private int age;

    // public getter to get the value everywhere
    public int getAge() {
        return this.age;
    }

    // setter to set the value for this field
    public void setAge(int age) {
        this.age = age;
    }
}

I added implements Comparable<Student> to the Student class because you mention you want to compare students by age. Also, check comments :

public class Student extends Human implements Comparable<Student> {
    // even though it extends Human - Student has no access to private
    // fields of Human class (you can declare it as protected if you want
    // your Student to have access to that field)

    // but protected does not guarantee it will be accessible everywhere!


    // now let's say you want to compare them by age. you can add implements Comparable
    // and override compareTo. getAge() is public and is inherited from the parent
    @Override
    public int compareTo(Student s) {
        return this.getAge() - s.getAge();
    }         
}

And your Group class needs something else. Because if this one is comparable - you compare groups, not students. And how you do it (I mean the rules like when group 1 is equal to group 2 and when it is less than group 2 etc.) - it's all up to you :)

public class Group implements Comparable<Group>  {
    private Student[] group = new Student[10];

    @Override
    public int compareTo(Group o) {
        // if your Group implements Comparable it means
        // you compare Groups not instances of class Student !
        // so here you need to implement rules for Group comparison !
        return .....
    }
}

Happy Hacking :)

Answer 2

检查是否为Human类中的age属性添加了get方法，并在compareTo方法中从o.getAge更改为o.getAge（）;